Recently, many people study an optical frequency comb signal generator which can generate plurality of optical frequency components with same frequency intervals simultaneously. An optical frequency comb signal generator, which can generates an optical frequency comb signal, can be used as one component of short pulse generation apparatus for ultra high speed optical communication or optical measurement. Further, an optical frequency comb signal generator is investigated its applications such as wideband multiwavelength laser sources, ultrashort pulse generation, arbitrary-waveform generation, and optical frequency reference (W. D. Jemison et al., MWP'01, pp. 169-172, 2001). An optical frequency comb signal generator is investigated to generate control signals of array antenna for space measurement system like an astronomical telescope (J. M. Payne and W. P. Shillue, MWP'02, pp. 9-12, 2002).
An optical frequency comb signal generator which has a mode lock laser using a semiconductor or an optical fiber, and an optical frequency comb signal generator which utilizes optical modulation technique using a LiNbO3 modulator are known in the art (e.g. non-patent documents 1 to 3).
Especially, a LiNbO3 modulator is suitable for the technologies because the bandwidth of its EO effect is much wider than that of the driving radio-frequency (rf) signal and tolerant to its large-amplitude operation. However, it is difficult to flatly generate an optical frequency comb using the EO modulation technique because the intensity of each mode is governed by Bessel functions and highly depends on wavelength.
Preceding reports have thus far solved this problem by using a two-stage modulator, where a phase modulator and an intensity modulator are cascaded in tandem. The optical frequency comb signal generator of the technology drives both of the optical phase modulator and the optical intensity modulator using the synchronize signals having the same frequency. The optical phase modulator is driven by large amplitude signals to generate high order components. Then the optical intensity modulator modulates intensity of the components. Sideband components generated from each of the frequency components of phase modulated signals interfere with neighboring order frequency component to lessen the deviation of the optical signal. Further, in time axis, nonlinear chirped components of the phase modulated light are selectively generated by controlling the optical intensity.
However the conventional optical frequency comb signal generator has a drawback that it requires the synchronized driving for two optical modulators connected in tandem. Thus the structure of the optical frequency comb signal generator become complex.